Interpretive Summary: Low, medium, and high numbers of the alfalfa leafcutting bee were released over four years in three research plots of Utah alfalfa planted at seed-production rates. A low number of bees (46-79% of released) survived the incubation and field emergence process, and of those bees, the number of females that established at the nesting sites was 25-100%. Of the three field sites used, more bees stayed and reproduced better at one site than the others, while at the others, not as many females stayed to nest. Also, the number of flowers decreased over time, although there were many that did not get visited over the season. Nest production decreased over time, as numbers of flowers and female bees declined. Significant positive relationships were found between the intended stocking densities and 1) the number of females that actually survived the incubation and field emergence and 2) the number of females that established nests at field domiciles. The number of females that established nests was significantly affected by the number of females that survived to emerge in the field. The percent of females that established was not significantly affected by the intended number of released bees, countering a prediction that the release of fewer bees would allow more females to establish nests and achieve high reproductive success. For growers, it may be more frugal to use modest numbers of alfalfa leafcutting bees for pollination, because many of the bees at medium and high numbers (30,000 and 50,000 bees per acre) do not stay at field sites.

Technical Abstract:
Low, medium, and high stocking densities of Megachile rotundata, the alfalfa leafcutting bee, were released over four years in three research plots of Utah alfalfa planted at seed-production rates. A low number of bees (46-79% of released) survived the incubation and field emergence processes, and of those bees, the number of females that established at the nesting sites was 25-100%. Of the three field sites used, one consistently retained more females and produced more completed nests than the others, while the others usually had poor female establishment and progeny production. Also, floral resources were depleted over time, but many flowers remained unvisited over the season. Nest production decreased over time, as numbers of flowers and female bees declined. Significant positive relationships were found between the intended stocking densities and 1) the number of females that actually survived the incubation and field emergence and 2) the number of females that established nests at field domiciles. The number of females that established nests was positively and significantly affected by the number of females that survived to emerge in the field. The percent of females that established was not significantly affected by the intended number of released bees, countering a prediction that the release of fewer bees would allow more females to establish nests and achieve high reproductive success. For growers, it may be more frugal to use modest numbers of M. rotundata for pollination, because many of the bees at medium and high stocking densities fail to be maintained at field sites.